1. Technical Field
The present invention relates to a semiconductor memory device and a mask pattern for defining the same, and more particularly, to a storage node electrode array of a semiconductor memory device and a mask pattern for defining the same.
2. Description of Related Art
As the integration density of semiconductor devices increases, the area occupied by a unit active cell decreases. Since the driving capability of a memory device, such as a DRAM, is determined by the capacitance of a capacitor, various efforts have been made to increase the capacitance of a capacitor irrespective of the decrease in the area required for the capacitor. Up to now, a concave type storage electrode has been most widely used in order to the increase the effective size of the capacitor.
FIG. 1 is a plan view of a conventional concave type storage electrode. Referring to FIG. 1, a plurality of concave type storage node electrodes 12 are arranged on a semiconductor substrate 10 that includes a MOS transistor (not shown) and other electric plugs (not shown). In other words, the concave type storage node electrodes 12 are horizontally and vertically arranged a predetermined distance apart in a sort of matrix. Thus, the storage node electrodes 12 belonging to the same row are arranged in a straight line. Also, the storage node electrodes 12 belonging to the same column are arranged in a straight line. Each of the concave type storage electrodes 12 may be oval-shaped.
However, as the integration density of memory devices increases, the density of the storage node electrodes 12 of a semiconductor memory device increases. Accordingly, it is difficult to maintain sufficient gaps S1 and S2 between adjacent storage node electrodes 12. If the gaps S1 and S2 between adjacent storage node electrodes 12 are not sufficiently maintained, as illustrated in FIG. 1, a bridge 14 may be generated between the storage node electrodes 12 causing occurrences of electrical defects, such as a twin-bit fail or a multi-bit fail.
There are many reasons why a bridge 14 may form. However, hereinafter, only the physical and dynamic reasons for the generation of the bridge 14 will be described. Referring to FIG. 1, concave type storage node electrodes 12 are formed on the semiconductor substrate, and the resultant substrate is cleaned. In the cleaning process, a water screen (not shown) may be formed between the storage node electrodes 12 due to a rinse. Then, oxygen O2 in the air easily dissolves in the water screen. The oxygen dissolved in the water screen forms a silicon oxide (SiO2) layer on the surface of the storage node electrodes 12. The silicon oxide layer is dissolved into a type of silicate in the water screen between the storage node electrodes 12. After that, a drying process is performed. Then, the volume of the water screen between the storage node electrodes 12 decreases, and thus surface tension between the storage node electrodes 12 increases. As a result, solid type silicate 14 only remains in the water screen between the storage node electrodes 12, and the remaining silicate 14 becomes a bridge.
The reason for the occurrence of the bridge 14 will be described more fully with reference to Equation (1). In general, as shown in FIGS. 2a and 2b, the forces in action between the two adjacent storage node electrodes are surface tension (Fs), which is an attractive force, and a shear-and-bending force (Fe), which is a repulsive force. Assuming that the storage node electrode 12 has a hexagonal structure and is a rigid beam installed on a hard substrate, the shear-and-bending force Fe can be expressed by Equation (1).                     Fe        =                              3            ⁢            EIx                                H            3                                              (        1        )            
In Equation (1), E is Young""s coefficient, that is, an elasticity coefficient of a material forming a storage node electrode, I is the inertial momentum of the horizontal cross-section of the storage node electrode 12, that is, the momentum for the storage node electrode 12 to continuously rotate in a spin dry process and is described as the elasticity of the storage node electrode 12 with respect to the thickness of the cylindrical storage node electrode 12, H is the height of the storage node electrode 12, and x is the distance by which the storage node electrode 12 is deformed. The deformation distance x is the distance between the original position of the upper part of the storage node electrode and the position of the upper part of the deformed storage node electrode.
The surface tension Fs between the storage node electrodes 12 is expressed by Equation (2).
Fs=2xcex3 sin xcex8(L+H)xe2x80x83xe2x80x83(2)
In Equation (2), y indicates the surface tension coefficient of water, and xcex8 indicates a contact angle which the storage node electrode 12 forms with water. L indicates the length of the storage node electrode 12, specifically, the opposing surface length of each of the two adjacent storage node electrodes.
In a state of equilibrium, the surface tension Fs and the shear-and-bending force Fe have the same strength. Thus, the deformation distance x of the storage node electrode 12 can be defined by Equation (3) obtained by combining Equation (1) and Equation (2).                     x        =                              2            ⁢                          xe2x80x83                        ⁢            γ            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          θ              ⁡                              (                                  L                  +                  H                                )                                      ⁢                          H              3                                            3            ⁢            EI                                              (        3        )            
According to Equation (3), the deformation distance x is proportional to the correspondence length L and the height H of the storage node electrode 12 but is inversely proportional to the elasticity coefficient E and the inertia momentum I of the storage node electrode 12.
Generally, the probability P of a bridge occurring between the storage node electrodes 12 is proportional to the deformation distance of each of the storage node electrodes 12 but is inversely proportional to the distance D between the storage node electrodes 12. Accordingly, these relations can be expressed by Equation (4) obtained by substituting the deformation distance x for the probability of bridge occurrence P in Equation (3).                     P        ∝                              2            ⁢                          xe2x80x83                        ⁢            γ            ⁢                          xe2x80x83                        ⁢            sin            ⁢                          xe2x80x83                        ⁢                          θ              ⁡                              (                                  L                  +                  H                                )                                      ⁢                          H              3                                            3            ⁢            EID                                              (        4        )            
As shown in Equation (4), as the distance D between the storage node electrodes 12 decreases, the height H of each of the storage node electrodes 12 increases, the opposing surface length L of each of the two horizontally-adjacent storage node electrodes 12 increases, and the bridge occurrence probability P increases.
However, to enhance the storage capacity of currently-used memory devices, a plurality of storage node electrodes must be integrated into a limited space and, simultaneously, the surface area of each of the storage node electrodes must be increased by increasing the height of each of the storage node electrodes. Thus, there is a limit to decreasing the distance between the storage node electrodes and the height and length of the storage node electrodes. Therefore, the probability of a bridge occurring becomes very high.
In addition, as shown in FIG. 3, to increase the surface area of each of the storage node electrodes 12 more, excessive light exposure must be performed to define the storage node electrodes 12. However, in this case, the storage node electrodes are defined to be larger than the desired storage node electrodes because of the excess exposure. Since the distance between the storage node electrodes 12 is very small, the adjacent storage node electrodes may contact together due to the excess exposure. Thus, it is difficult to perform an additional process to increase the surface area of the storage node electrodes 12. Reference numeral 15 of FIG. 3, which is not yet mentioned, indicates the increased area of an additionally expanded storage node electrode caused by excess exposure.
To solve the above and other related problems of the prior art, there is provided a semiconductor memory device having improved electrical characteristics. The semiconductor memory device according to the present invention is capable of reducing the probability of a bridge being generated between storage node electrodes.
According to an aspect of the present invention, there is provided a semiconductor memory device including a plurality of storage node electrodes that are vertically and horizontally arranged a predetermined distance apart in columns and rows, respectively. Among the plurality of storage node electrodes, storage node electrodes belonging to even-numbered columns are shifted up or down a predetermined distance.
According to another aspect of the present invention, the shifted storage node electrodes are shifted by a same distance.
According to yet another aspect of the present invention, the shifted storage node electrodes are shifted in a gap between vertically adjacent storage node electrodes belonging to a same column.
According to still another aspect of the present invention, the storage node electrodes are oval-shaped concave type storage node electrodes.
According to still yet another aspect of the present invention, there is provided a mask pattern for forming storage node electrodes. The mask pattern comprises a transparent substrate. A plurality of blocking layers are formed on the transparent substrate in rows and columns for defining shapes of the storage node electrodes. Among the plurality of blocking layers, blocking layers belonging to even-numbered columns are shifted up or down an equal distance.
According to a further aspect of the present invention, the shifted blocking layers are shifted in a gap between vertically adjacent blocking layers belonging to a same column.
According to a still further aspect of the present invention, the plurality of blocking layers are rectangular.
According to yet a still further aspect of the present invention, the mask pattern further comprises serifs for light compensation. The serifs are attached to at least one side of ends of the shifted blocking layers opposite to a direction in which the shifted blocking layers are shifted and to at least one side of ends of non-shifted blocking layers corresponding to the direction in which the shifted blocking layers are shifted.